A normal human auditory system can not discriminate the time difference for the transmitted signal of a transmitting transducer to travel out to an object and back again by way of a receiving transducer, since the ultrasonic signal is propagated at the speed of about 340 meters per second, the relevant time difference is several milliseconds and the frequency of the ultrasound signal is beyond the human audible range.
Accordingly, the reflected echo signal which is received from an object in the field of view has to be converted into an audible frequency in order that a blind person can discriminate the echo signal.
In the prior art signal processing method disclosed in U.S. Pat. No. 3,366,922, the sonic aid for a blind has utilized a method that processes a reflected echo signal corresponding to an audible frequency proportional to the time delay of said echo signal. For example, it takes about 6 msec time delay for the transmitted ultrasound to travel out to an object located one meter ahead and back again. It is because, in such a case, a person cannot sense the reflected echo signal at such a time delay. The prior art signal processing method has converted the reflected echo signal received from an object which is located one meter ahead into an audible frequency of 1000 Hz in succession, and has converted the echo signal received two meters ahead into an audible frequency of 2000 Hz in succession, and has thereby detected the position of an object. Therefore, the prior art signal processing method has been defective in that, with such manner of converting successively the reflected echo signal comprising the time difference proportional to the range of an object into an audible frequency, it is hard for a blind person to discriminate respectively the position of the objects due to the successively incoming mixed signal received from one and other objects, and has also been defective in that, since the audible frequency varies in accordance with the several positions even in the same object, it is hard for a blind person to recognize the object. And the conventional blind aid which is utilized in the above method has been defective in that, since the frequency discriminating capability of a human auditory system is not to provide recognization as by the absolute difference of the frequency but the relative difference of the frequency, and has also been defective in that, the transmitting transducer needs the wide band characteristics of 40 KHz to 80 KHz at which the ultrasonic oscillator generates, it does not make use of the narrow band transducer which is cheap and of good performance.